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Abstract

Electrochemical micromachining (ECMM) is a non-conventional manufacturing method suitable for the production of microsized components on a wide range of conductive materials. ECMM improves dimensional accuracy and simplifies tool design for machining hard, high strength, heat resistant, and conductive materials into complex shapes. Extremely small interelectrode gaps of the order of few microns are required in ECMM for better dimensional accuracy. However, excessively small interelectrode gaps may lead to complications, such as short-circuiting, which disrupt the stability of ECMM process. This necessitates the need for better understanding of the interelectrode gap dynamics. This paper presents a mathematical model for the analysis of interelectrode gap under non-steady state conditions in micromilling of steel using the ECMM process. Experimental verification of the mathematical model was conducted using an in-house built micro electrochemical machining system. The model is capable of predicting the machining results to within 1- 5 µm error (10- 50%).

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Introduction

Micro manufacturing has taken center stage with the advent of product miniaturization. There is an ever-growing demand for micro sized parts and novel methods to produce them. Electrochemical Machining (ECM) is one of the emerging micromachining processes that has the potential to meet this demand. The introduction of pulsed current to the process of ECM has enabled applications in microfabrication, with nano scale precision.

Electrochemical machining (ECM) is a targeted anodic electrochemical dissolution process. The removal of material occurs atom by atom from the workpiece surface (anode) by the tool (cathode). ECM has many advantages over other traditional machining processes. Some of which are its applicability regardless of material hardness, absence of tool wear, high material removal rate, smooth and bright surface, and production of geometrically complex components (Rajurkar, Zhu, McGeough, Kozak, & De Silva, 1999). ECM is an effective method for producing a wide variety of parts for the aerospace, automotive, defense, and medical industries. These parts include turbine blades, engine castings, bearing cages, gears, dies and molds, artillery projectiles, and surgical implants (Rajurkar et al., 1999). Pulse electrochemical micromachining (PECMM) is a variation of ECM where instead of using direct current, a pulsed current is used as the power source. PECMM provides a superior alternative to ECM and many other traditional machining processes. Compared to direct current, pulsed current yields improved electrolyte flow in the interelectrode gap, enhanced localization of anodic dissolution, and smaller and more stable gaps. This leads to higher dimensional accuracy, better process stability, relatively simpler tool design and better suitability for online process control (Rajurkar et al., 2006). The PECMM process has been used in the fabrication of micro features (eg: holes, slots, cylinders) on several materials (Jain, Kalia, Sidpara, & Kulkarni, 2012; Kim, Na, Lee, Choi, & Chu, 2005; Mathew, James, & Sundaram, 2010; Mithu, Fantoni, & Ciampi, 2011).

There are two major variations in ECM, depending on the mode of machining, in order to achieve the required machined surface. The first variation is ECM sinking in a steady state process. In this process, the tool profile is a 3-D negative image of the required surface profile. The tool is allowed to sink in to the workpiece at a constant feed rate until the required shape is obtained on the workpiece. Models developed for this process concentrate on the prediction of the equilibrium gap, which is necessary to design the actual tool profile. Another variation is the ECM shaping process. In this process, a universal, simple shaped tool (e.g. cylinder) is moved along a specified path to obtain the required shape of the workpiece. These two process variants are illustrated in Figure 1.

Figure 1.

Electrochemical shaping with shaped (left) and unshaped tool (right)

Aside from the elimination of expensive electrodes, one of the most important advantages of the ECM with unshaped tool is the increase in machining accuracy and workpiece surface quality. This improvement is achieved by the decrease of the working area of the electrode, significantly reducing the influence of heat and gas generation on the electrolyte properties in the interelectrode gap. This reduction causes the conditions of dissolution to be more uniform and allows machining with a smaller interelectrode gap (Rajurkar et al., 2006). Depending on which of the electrode surface needs to be determined/modeled, all the models in ECM can be classified into two categories:

1.

Models in which for, a known shape of the tool electrode, and known condition of machining, the evolution of the shape of workpiece surface has to be determined;

2.

Models in which the tool electrode shape is searched for, which ensures obtaining the required shape of the workpiece (Jerzy Kozak, Budzynski, & Domanowski, 1998).